Sadly for your friend, the neighbor doesn't have to have paid any taxes whatsoever on the property if he's claiming it under adverse possession.
However, tell your friend not to give up so easily! As I mentioned before, you friend may have a good case.
IF your friend can show the court he complained to the neighbor (the sheriff could be compelled to be your friend's witness) --AND that he did it before the neighbor had used the land for the period named by Arkansas law (either 15 or 7 years, depending on the kind of property)-- then the neighbor doesn't have a claim.
No matter what happens, my friend is going to have to spend a lot of money and he doesn’t have it. The neighbor knows my friend is poor and can’t afford a legal fight. That is the way he will take over the land.